1. Field of the Invention
The present invention relates to a glass substrate and a method of making a glass substrate usable for a semiconductor device, which requires high temperature processing.
2. Description of the Prior Art
Up to the present, glass substrates for semiconductor devices have had almost zero value of thermal expansion coefficient at temperatures of 300.degree.-600.degree. C., which are necessary to make a semiconductor device. The crystallized glass is obtained by growing a crystal in the glass which has been previously heated to form a crystal nucleus. It is possible to make a glass which is resistant to thermal impact by controlling the crystallization to adjust the thermal expansion coefficient of the glass.
In general, glass will expand by heating. This results from the fact that glass has a positive coefficient of the thermal expansion. But, it is possible to make the thermal expansion coefficient at some temperatures approach zero without being plus or minus over the entire part of glass, by depositing crystallized constituents having negative coefficients of thermal expansion with the constituents of glass.
Products such as Liquid Crystal Display Devices and Image Sensors are produced by directly forming a semiconductor device, like a thin film transistor, on a glass substrate. However, to make such a semiconductor device, heating of more than 600.degree. C. is required, which may cause glass to shrink. The glass shrinkage will shorten the glass substrate after glass is heated and then cooled. This phenomenon will occur repeatedly, even after a substrate is once shrunken. In fact, it will shrink more by a second thermal treatment. Namely, although it is generally known that glass will expand by heating, it has a hysteresis which causes glass to shrink compared with its original measure. The shrinkage ratio will be lowered in accordance with repeated thermal treatment.
The problem that the glass substrate will expand in the heating process can be solved by using the above-mentioned crystallized glass. For instance, as it is known that glass is controlled with the thermal expansion coefficient, heat resistant glass has been put into practical use.
On the other hand, the aforementioned problem of glass substrate shrinkage has caused difficulty using a mask combination in the making of a semiconductor device such as an Insulated Gate Field Effect Transistor (hereinafter to be referred to as TFT). That is, many sheets of photomask are used while processing a TFT. The masks in every process are different from each other in their pattern shapes, but they are designed to constitute the same TFT after all the processing is finished. As a matter of course, all the masks are prepared according to the same dimensional standards.
In the process of making the TFT using such masks, it is necessary to heat the substrate once at least. For that reason, the substrate will shrink before and after the heating process. As a result, some of patterns will not coincide with the substrate when the masks are made according to the same dimensional standard.
To overcome this problem of glass substrate shrinkage during the heating process, it has been proposed to shrink a glass substrate in advance by thermal treatment. This is useful in lightening the shrinkage of a substrate to some extent. The shrinkage of a glass substrate by heating is caused by the fact that an amorphous constituent existing in a glass substrate will be crystallized by heating. As a result, a volume of the substrate will be decreased. Accordingly, this proposed method is designed to remove the utmost of the constituents to be crystallized by heating, which is effected in an actual use of a glass substrate, by crystallizing in advance the amorphous constituents in glass.
However, it has not been practical to cause beforehand shrinkage of a glass substrate by heating in advance, as stated above, since it is not easy to stop complete shrinkage. Especially, in such case of an active Liquid Crystal Display Device having a big area or a contact type Image Sensor of the A4 or B4 size. In these cases, it is needed to fairly decrease the shrinkage of a glass substrate, since the required dimension of a substrate is very big. As a result, it has been impossible to attain a sufficiently effective method of one or two thermal treatments in advance.
Also, in the case where a semiconductor device includes a silicon semiconductor film with a crystalline character, an amorphous silicon on a glass substrate is crystallized by heating. In this case, a problem exists between the shrinkage difference of a silicon semiconductor film and a glass substrate, resulting in needless stress to the silicon film having a crystalline character which may cause cracking in the crystalline silicon.